PLL dual edge lock detector

ABSTRACT

A lock signal indicating that a target signal is in phase with a reference signal includes detecting the reference signal at the rising and falling edges of the target signal. The target signal is detected on the rising and falling edges of the reference signal. An out of phase condition between the target and reference signals is used to place a timing means in a reset state. When the timing means is allowed to time out, a signal is asserted which indicates that the target signal is deemed to be locked to the reference signal.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure claims priority to U.S. Provisional App. No. 61/406,953 filed Oct. 26, 2010, the content of which is incorporated herein by reference in its entirety for all purposes.

BACKGROUND

The present disclosure relates to phase locked loop (PLL) circuits and in particular to circuitry for detecting when the output of the PLL has locked onto a reference signal.

Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

Phase locked loops (PPLs) are widely used in communications electronics and digital electronics. In wireless products, PLLs are typically used to generate high speed system clocks. When the electronics for a PLL are powered up, it can take on the order of tens of microseconds for the frequency of the PLL output signal to become stable. A lock detector circuit is typically provided to detect when the frequency of the PLL output signal has settled. The lock detector can generate a lock signal that can be used by the system for clock gating.

A typical PLL circuit configuration is illustrated in FIG. 1. The main blocks of a PLL circuit 100 typically include a phase frequency detector 112, a charge pump 114, a low-pass filter 116, a voltage controlled oscillator (VCO) 118, and a feedback counter 120. The phase frequency detector 112 detects the difference in phase and frequency between a reference signal (clock) F_(ref) and a feedback signal (clock) F_(fb), and generates an “up” (U) or a “down” (D) control signal based on whether the frequency of the feedback signal F_(fb) (feedback frequency) is lagging or leading the frequency of the reference signal F_(ref) (reference frequency).

The phase frequency detector 112 outputs these “up” and “down” signals to the charge pump 114. If the charge pump receives an “up” signal, current is driven into the low-pass filter 116. Conversely, if the charge pump 114 receives a “down” signal, current is drawn from the low-pass filter 116. The low-pass filter 116 converts these signals to a control voltage (a DC level) that is used to control the VCO 118.

Based on the control voltage, a PLL output signal F_(o) of the VCO 118 oscillates at a higher or lower frequency, which affects the phase and frequency of the feedback signal F_(fb). Thus, if the phase detector 112 produces an “up” s signal, then the frequency of PLL output signal F_(o) increases. A “down” signal decreases the frequency of the PLL output signal F_(o). The output of the VCO 118 stabilizes once the reference signal F_(ref) and the feedback signal F_(fb) have the same phase and frequency. When the reference signal F_(ref) and the feedback signal F_(fb) are aligned, the PLL output signal F_(o) is referred to variously as “locked,” “locked to the reference signal,” and the like.

A divide-by-N counter 120 may be inserted in the feedback loop to increase an amplitude of the PLL output signal F_(o) above the reference signal F_(ref). The PLL output signal F_(o) is equal to N times the reference signal F_(ref).

A lock detector 102 compares the reference signal F_(ref) and the feedback signal F_(fb) to determine when a lock condition has occurred. A lock signal is output when the lock condition has occurred.

FIG. 1 further illustrates a simple timing chart. When power is applied to the circuitry at time t0, the frequency of the PLL output signal F_(o) begins to increase. During a period of time between t0 and t1, the PLL frequency is not unstable and cannot be used by the system. At time t1, the PLL frequency has stabilized, but typically a period of time (t2-t1) is allowed to pass to ensure that the PLL output signal F_(o) has fully settled. Accordingly, during the period of time between t0 and t2, the lock signal remains “LO”. At time t2, the lock signal goes “HI” to signify that the PLL output signal F_(o) is ready to clock the system logic.

FIG. 2 illustrates a conventional implementation for the lock detector 102 which determines signal lock based on the rising edges of the reference signal F_(ref) and the feedback signal F_(fb). The lock detector comprises two flip flop circuits, such as the flip flops 202 and 204. The reference signal F_(ref) is delayed by an inverted delay line 206, and the delayed signal F_(ref)′ feeds into the flip flop 202. The feedback signal F_(fb) clocks the flip flop 202. Similarly for the flip flop 204, the feedback signal F_(fb) is delayed by an inverted delay line 208, and the delayed signal F_(fb)′ feeds into the flip flop 204. The feedback signal F_(ref) clocks the flip flop 204. The outputs of the flip flops 202 and 204 feed into NAND gate 210.

A timing circuit 212 comprises a capacitor 222 that is charged by a current source 224. So long as transistor 228 remains in the OFF condition, then after a certain amount of time proportional to I/C, where I is the current and C is the capacitance, the capacitor 222 will charge to a voltage level (i.e., a threshold voltage level) sufficient to trigger a buffer 228 and output a signal that constitutes the lock detect signal. The transistor 228 is controlled by the output of the NAND gate 210. During power up, the phase between the reference signal F_(ref) and the feedback signal F_(fb) will vary until the feedback signal attains a lock with the reference signal.

Referring to FIG. 3, a timing diagram of the reference signal F_(ref) and the feedback signal F_(fb) is illustrated for the lock condition. In this and subsequent figures, the shaded areas in the timing diagrams represent prior output states (“HI” or “LO”) that depend on the previous phase relation of the signals F_(ref) and F_(fb). The flip flops 202 and 204 shown in FIG. 2 are clocked on the rising edges of F_(fb) and F_(ref) respectively. Accordingly, the flip flops will latch respective inverted signals F_(ref)′ and F_(fb)′ delayed by τ (the propagation delay of inverted delay lines 206 and 208). As can be seen, in the lock condition depicted in FIG. 3, the rising edges of the reference signal F_(ref) and the feedback signal F_(fb) are aligned. The flip flop outputs Q₂₀₂ and Q₂₀₄ are “HI”, and consequently the output of NAND gate 210 is “LO”. Thus, the transistor 228 is OFF and will remain in the OFF state so long as the lock condition exists, allowing the capacitor 222 to continue charging and the lock detect signal asserted.

Referring to FIG. 4, a timing diagram is illustrated for a “no lock” condition where the reference signal F_(ref) and the feedback signal F_(fb) are not in phase. The timing diagram shows that the output of flip flop 202 is “LO” because of the timing of the rising edge of F_(fb) relative to the inverted and delayed reference signal F_(ref)′. Accordingly, the NAND gate 210 is “HI” for this condition. The transistor 228 is turned ON and consequently the capacitor 222 does not charge.

Referring to FIG. 5, a timing diagram is illustrated for a “no lock” condition, where the phase relation between the reference signal F_(ref) and the feedback signal F_(fb) create a condition at time t_(x) that causes the NAND gate 210 to incorrectly outputs a “LO” logic level. The “LO” output of the NAND gate 210 turns OFF the transistor 228, allowing the capacitor 222 to charge and the lock detect signal to erroneously assert.

SUMMARY

In some embodiments, a circuit includes inputs for receiving a reference signal and a target signal. The circuit includes first circuitry to output first levels of the reference signal taken at the rising and falling edges of the target signal. Second circuitry outputs second levels of the target signal taken at the rising and falling edges of the reference signal. Third circuitry asserts a third level when the first and second levels indicate that the target signal is not in phase with the reference signal.

In some embodiments, fourth circuitry outputs a fourth signal after an amount of time has passed, and is operative to be reset in response to occurrence of the third level. The fourth circuitry may include a counter that includes a reset input connected to the third circuitry. In other embodiments, the fourth circuitry may include a current source, a transistor, and a capacitor. The capacitor is charged by a current from the current source, and can be discharged by the transistor. In other embodiments, the fourth circuitry may include a digital processing unit executing firmware.

In some embodiments, the first circuitry includes a delay element to output a delayed reference signal, wherein the first levels are based on the delayed reference signal. In some embodiments, the second circuitry includes a delay element output a delayed target signal, wherein the third and fourth levels are based on the delayed target signal.

In an embodiment, the circuit further includes a phase locked loop (PLL) circuit having an input to receive the reference signal and having a PLL feedback signal based on an output of the PLL circuit, wherein the PLL feedback signal is the target signal.

In some embodiments, a method of operating a circuit includes receiving a reference signal and receiving a target signal. First levels of the reference signal are generated at the rising and falling edges of the target signal. Second levels of the target signal are generated at the rising and falling edges of the reference signal. An operation is performed which includes asserting a lock signal after an amount of time has passed. The operation is restarted when the first and second levels indicate that the target signal is not in phase with the reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show a prior art lock detector.

FIGS. 3-5 illustrate timing charts based on the prior art lock detector.

FIG. 6 shows an illustrative embodiment of a lock detector in accordance with principles of the present disclosure.

FIG. 6A illustrates the use of firmware for the timing circuitry.

FIGS. 7-9 show timing diagrams for various phase relations between the reference signal and the target signal in accordance with principles of the present disclosure.

FIG. 10 shows an embodiment of a lock detector of the present disclosure using an analog-based timing circuit.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be evident, however, to one skilled in the art that the present disclosure as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

A lock detector circuit in accordance with principles of the present disclosure can provide an indication of a lock condition between a reference signal and a target signal. In some embodiments, the reference signal may be a reference signal in a phase locked loop (PLL) circuit and the target signal may be a feedback signal in a feedback loop of the PLL circuit. An example of a PLL circuit is illustrated in FIG. 1. The lock detector circuit in accordance with principles of the present disclosure, however, can be used in any circuitry where it is desired to determine if one signal (e.g., the target signal) has achieved a lock with another signal (e.g., the reference signal).

Referring to FIG. 6, in some embodiments, a lock detector 600 may comprises flip flops 602, 604, 606, and 608. As will become clear later on, the flip flops 602 and 604 constitute a dual edge-triggered detector. Similarly, flip flops 606 and 608 also constitute a dual edge-triggered detector. In the particular embodiment shown in FIG. 6, the flip flops 602, 604, 606, and 608 are D-type flip flops. It will be appreciated, however, that other types of flip flops may be substituted.

The lock detector 600 receives a reference signal F_(ref) and a target signal F_(target), and may output a lock detect signal. The reference signal F_(ref) feeds into an inverted delay line 610. The inverted delay line 610 is associated with a propagation delay τ. The output of the inverted delay line 610 is an inverted signal F_(ref)′, which is delayed relative to the incoming reference signal F_(ref) by an amount based on the propagation delay τ. The inverted signal F_(ref)′ feeds into flip flops 602 and 604. The flip flops 602 and 604 are clocked by the target signal F_(target). In particular, the flip flop 602 is configured with a non-inverting clock input, and thus is triggered by the rising edges of the target signal F_(target); while the flip flop 604 is configured with an inverting clock input, and so is triggered on the falling edges of the target signal.

The target signal F_(target) feeds into an inverter 612 for detection, in addition to serving as a trigger. In some embodiments, the inverter 612 may have substantially the same propagation delay τ as inverter 610. The output of the inverter 612 is an inverted signal F_(target)′, which is delayed relative to the incoming target signal F_(target) by an amount proportional to τ. The inverted signal F_(target)′ feeds into flip flops 606 and 608. The flip flops 606 and 608 are clocked by the reference signal F_(ref). The flip flop 606 is configured with a non-inverting clock input, and thus is triggered on the rising edges of the reference signal F_(ref). The flip flop 608 is configured with an inverting clock input, and thus is triggered on the falling edges of the reference signal F_(ref).

It can be appreciated that any suitable means for detecting and holding a level of the reference signal F_(ref) in synchrony with the rising edge and falling edge of the target signal F_(target) may be used in place of the flip flops 602 and 604. Likewise, any suitable means for detecting and holding a level of the target signal F_(target) in synchrony with the rising edge and falling edge of the reference signal F_(ref) may be used in place of the flip flops 606 and 608.

In some embodiments, the lock detector 600 may further include a NAND gate 614 for receiving outputs Q₆₀₄ and Q₆₀₈ of respective flip flops 604 and 608. An output of NAND gate 614 feeds into an input of an AND gate 616. The outputs Q₆₀₄ and Q₆₀₈ represent levels, respectively, of the inverted and delayed signals F_(ref)′ and F_(target)′ detected on the falling edges of signals F_(target) and F_(ref), respectively.

The AND gate 616 receives input from flip flops 602 and 606, in addition to input from NAND gate 614. The output Q₆₀₂ from flip flop 602 and the output Q₆₀₆ from flip flop 606 feed into AND gate 616. The outputs Q₆₀₂ and Q₆₀₆ represent levels, respectively, of the inverted and delayed signals F_(ref)′ and F_(target)∝ detected on the rising edges of signals F_(target) and F_(ref), respectively.

In some embodiments, the lock detector 600 may further include a timing circuit 618. The timing circuit 618 operates for a given amount of time T from an initial state, and then asserts a signal at an output 618 a. The timing circuit 618 may include a reset line 618 b that resets the timing circuit to the initial state, thus restarting the timing operation from that initial state. In some embodiments, the amount of time T may be configurable. The output of AND gate 616 is connected to the reset line 618 b of the timing circuit 618.

In an embodiment, the timing circuit 618 may comprise a counter 622. For example, the counter 622 may count from an initial state of zero to some maximum count value. It will be appreciated that any of numerous implementations for a counter may be employed. An output of the counter 622 may serve as the output 618 a of the timing circuit 618. For example, the output may be asserted when the counter has reached its maximum count value. The counter 622 includes a reset input R (the figure shows an inverting reset input) that is connected to the reset line 618 b of the timing circuit 618, which in turn is connected to the output of AND gate 616. When the reset input R is asserted by the AND gate 616, the counter 622 resets to the initial state and restarts the counting sequence; e.g., starts counting from zero.

The counter 622 may be clocked using the reference signal F_(ref) and may be configured to assert a signal at its output after a maximum count value has been reached. Accordingly, the counter 622 can be configured to run for a suitable amount of time T by appropriately configuring the maximum count value. In some embodiments, the maximum count value may be programmable and hence the amount of time T can be programmed. In other embodiments, the maximum count value may be a fixed value.

Referring to FIG. 6A, in another embodiment, the lock detector 600 may employ a timing circuit 618′ that is implemented with firmware comprising executable program code executing on a suitable digital processing unit 622′, such as a general purpose IO (GPIO) processor. The reset line 618 b and the reference signal F_(ref) may be connected to input pins of the GPIO processor 622′. The output 618 a may be connected to an output pin of the GPIO processor 622′. The firmware can be programmed to respond to signals on the input pins to output a signal on the output pin to behave in the same manner as discussed above for timing circuit 618 shown in FIG. 6.

Operation of the lock detector 600 may be explained in connection with a timing diagram. Referring to FIG. 7, a timing diagram for the lock condition, where the target signal F_(target) is locked to (i.e., in phase with) the reference signal F_(ref), will be discussed. In this and subsequent figures, the shaded areas in the timing diagrams represent prior output states of the flip flops that depend on the previous phase relation of the signals F_(ref) and F_(target). However, since the figures do not show the details of the waveforms for F_(ref) and F_(target), the prior output states of the flip flops are not known and hence shaded.

The output Q₆₀₄ is “LO” when the flip flop 604 is triggered by the falling edge of the target signal F_(target) because the signal level of the inverted signal F_(ref)′ is “LO” at the time of the trigger. Likewise, the output Q₆₀₈ is “LO” when the flip flop 608 is triggered by the falling edge of the reference signal F_(ref) because the signal level of the inverted signal F_(target)′ is “LO” at the time of the trigger. Consequently, the output of NAND gate 614 is “HI”.

The output Q₆₀₂ is “HI” when the flip flop 602 is triggered by the rising edge of the target signal F_(target) because the signal level of the inverted signal F_(ref)′ is “HI” at the time of the trigger. Likewise, the output Q₆₀₆ is “HI” when the flip flop 606 is triggered by the rising edge of the reference signal F_(ref) because the signal level of the inverted signal F_(target) is “HI” at the time of the trigger. Since all three inputs to the AND gate 616 are “HI”, the AND gate asserts a “HI” level on the reset line 618 b of the timing circuit 618.

Since the reset input of the counter 622 is an inverted input, the counter does not reset and continues to count at rate based on the frequency of the clock signal, which in the embodiment shown in FIG. 6 is the reference signal F_(ref). When a period of time based on the maximum count value has passed, the counter 622 will assert a signal (e.g., “HI” level) on the output 618 a of the timing circuit 618.

It can be appreciated that the period of time can be adjusted depending on how much time is desired to determine that a lock condition has been achieved. Referring to FIG. 1, during power up, the feedback signal F_(fb) (i.e., the target signal) may enter into a period where its frequency fluctuates above and below the reference frequency (signal F_(ref)), and hence a lock has not been achieved. The period of time provided by the counter 622, therefore, should be long enough to avoid a false positive lock indication during those fluctuations.

It will be appreciated that timing charts similar to FIG. 7 can be used to arrive at the conclusion that other “no lock” (out of phase) conditions between F_(ref) and F_(target) will cause the timing circuit 618 to be held in a reset state. Thus, an out of phase condition between F_(ref) and F_(target) places the timing circuit 618 in reset and prevents it from timing out. The timing circuit 618 is enabled only when the F_(target) is locked with F_(ref,) where the timing circuit 618 can time out so that lock detect signal can be asserted, thus indicating that the target signal F_(target) is deemed to be locked to the reference signal F_(ref.) Consider, for example, FIG. 8 which shows a timing diagram of the lock detector 600 for the phase relation between F_(ref) and F_(target) shown in FIG. 4. As can be seen from the figure, the output of the AND gate 616 is “LO”, which will hold the counter 622 in reset.

FIG. 9 shows a timing diagram of the lock detector 600 for the “no lock” situation shown in FIG. 5, where a conventional lock detector gave a false positive indication of a lock. As shown in the figure, at time t_(x) the output of AND gate 616 outputs “LO” when the falling edge of the target signal F_(target) clocks the inverted signal F_(ref)′. Referring back to FIG. 6, “LO” logic level at the output of AND gate 616 resets the counter 622 in the timing circuit 618. As can be seen in FIG. 8, the AND gate 616 will maintain a “LO” output for the given phase relationship between F_(ref) and F_(target,) and thus will hold the counter 622 in reset. Consequently, where this no lock situation would have resulted in a false positive lock indication using the conventional lock detector 102 shown in FIG. 2, a lock detector in accordance with principles of the present disclosure will correctly indicate a no lock situation (e.g., by not asserting the lock detect signal).

In FIG. 6, the lock detector 600 employs a timing circuit 618 comprising a counter 622, which in some embodiments is a digital circuit. In other embodiments, a lock detector in accordance with principles of the present disclosure may employ an analog-based timing circuit. Thus in some embodiments, such as illustrated in FIG. 10, a lock detector 1000 may include an analog timing circuit 1018. For example, the timing circuit 1018 may employ a circuit similar to the circuit 212 shown in FIG. 2. The initial state of the timing circuit 1018 may be a fully discharged capacitor 1022. When the capacitor 1022 charges to a level above a trigger level of the buffer 1024, the buffer will assert a signal that constitutes the lock detect signal. The timing circuit 1018 may be reset to the initial state when the output of AND gate 616 asserts a signal that turns ON transistor 1026 to discharge the capacitor 1022. The charge time of the capacitor 1022 is proportional to the current flow I into the capacitor and the capacitance of the capacitor. The timing diagrams discussed in connection with FIG. 6 apply to timing of the lock detector 1000.

The digital counter such as the counter 622 in FIG. 6 uses smaller components (and thus is more efficient in terms of area) and may consume less power than the analog counterpart. The digital counter is more easily configurable. For example, the time can be varied by removing or adding counting stages (typically a flip flop) to the counter 622 without significant impact silicon area. Nonetheless, it will be appreciated that a lock detector in accordance with the principles of the present disclosure may employ the analog-based timing circuit 1018 instead of a digital circuit.

As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

The above description illustrates various embodiments of the present disclosure along with examples of how aspects of they may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present disclosure as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the claims. 

What is claimed is:
 1. A circuit configured to receive a reference signal and a target signal comprising: first circuitry operative to output first levels representative of the reference signal when triggered by a rising edge of the target signal and a falling edge of the target signal; second circuitry operative to output second levels representative of the target signal when triggered by a rising edge of the reference signal and taken at a falling edge of the reference signal; and third circuitry configured to receive the first levels and the second levels, respectively, from the first and second circuitry, and operative to output a third level when the first levels and the second levels indicate that the target signal is not in phase with the reference signal.
 2. The circuit of claim 1, further comprising fourth circuitry connected to the third circuitry and operative to output a fourth level after an amount of time has passed, wherein the fourth circuitry is further operative to reset when the third circuitry outputs the third level.
 3. The circuit of claim 1 wherein the first circuitry comprises a first delay element to output a delayed reference signal, wherein the first levels are based on the delayed reference signal.
 4. The circuit of claim 3 wherein the second circuitry comprises a second delay element to output a delayed target signal, wherein the second, third and fourth levels are based on the delayed target signal.
 5. The circuit of claim 1 wherein the third circuitry is further operative to output a fifth level different from the third level when the target signal is in phase with the reference signal, wherein the fourth circuitry does not reset when the third circuitry outputs the fifth level.
 6. The circuit of claim 1 wherein the first circuitry comprises: a first circuit operative to output a first detected level representative of the reference signal detected in synchrony with the rising edge of the target signal; and a second circuit configured to output a second detected level representative of the reference signal detected in synchrony with the falling edge of the target signal, wherein the first levels comprise the first detected level and the second detected level.
 7. The circuit of claim 6 wherein the second circuitry comprises: a third circuit configured to output a third detected level representative of the target signal detected in synchrony with the rising edge of the reference output signal; and a fourth circuit configured to output a fourth detected level representative of the target signal detected in synchrony with the falling edge of the reference output signal, wherein the second levels comprise the third detected level and the fourth detected level.
 8. The circuit of claim 1 wherein the fourth circuitry comprises a timing circuit having a reset input connected to an output of the third circuitry, or firmware executing on a digital processing unit.
 9. The circuit of claim 1 wherein the fourth circuitry comprises a current source, a transistor, and a capacitor, wherein the capacitor is charged by a current from the current source, wherein a control gate of the transistor is connected to an output of the third circuitry and is configured to discharge the capacitor when the transistor is turned on, wherein the fourth level is based on a voltage level of the capacitor.
 10. The circuit of claim 1 further comprising a phase locked loop (PLL) circuit having an input to receive the reference signal and having a PLL feedback signal based on an output of the PLL circuit, wherein the PLL feedback signal constitutes the target signal.
 11. A circuit configured to receive a reference signal and a target signal comprising: first means for detecting a first level and a second level of the reference signal based respectively on a rising edge and a falling edge of the target signal; second means for detecting a first level and a second level of the target signal based respectively on a rising edge and a falling edge of the reference signal; timer means for generating a lock signal after an amount of time has elapsed; and reset means for resetting the timer when the first and second levels of the reference signal and the first and second levels of the target signal indicate that the target signal is not in phase with the reference signal.
 12. The circuit of claim 11 wherein the lock signal indicates that the target signal is in phase with the reference signal.
 13. The circuit of claim 11 wherein the first means includes means for delaying the reference signal relative to the target signal.
 14. The circuit of claim 11 wherein the second means include means for delaying the target signal relative to the reference signal.
 15. The circuit of claim 11 wherein the timer means comprises a current source, a transistor, and a capacitor, wherein the transistor is configured to charge the capacitor and to discharge the capacitor depending on a control signal that is based on the first level, the second level, the third level, and the fourth level, wherein the lock signal is based on a voltage level of the capacitor.
 16. The circuit of claim 11 wherein the timer means comprises a counter.
 17. The circuit of claim 11 further comprising a phase locked loop (PLL) circuit having an input to receive the reference signal and having a PLL feedback signal based on an output of the PLL circuit, wherein the PLL feedback signal constitutes the target signal.
 18. A method in a circuit comprising: receiving a reference signal; receiving a target signal; generating first levels representative of the reference signal at the time of a rising edge of the target signal and at the time of a falling edge of the target signal; generating second levels representative of the target signal at the time of a rising edge of the reference signal and at the time of a falling edge of the reference signal; performing an operation that includes asserting a lock signal after a predetermined amount of time has passed; and selectively restarting the operation when the first and second levels indicate that the target signal is not in phase with the reference signal.
 19. The method of claim 18 further comprising delaying the reference signal prior to generating the first levels.
 20. The method of claim 19 further comprising delaying the target signal prior to generating the second levels. 